In an air-conditioning apparatus, such as a multi-air-conditioning apparatus for a building, refrigerant is circulated between an outdoor unit, functioning as a heat source unit, disposed outside a structure, for example, and an indoor unit disposed inside an indoor space of the structure. The refrigerant transfers heat or removes heat to heat or cool air, thus heating or cooling a conditioned space through the heated or cooled air. As regards the refrigerant, for example, an HFC (hydrofluorocarbon) refrigerant is often used. An air-conditioning apparatus using a natural refrigerant, such as carbon dioxide (CO2), is also proposed.
Furthermore, in an air-conditioning apparatus called a chiller, cooling energy or heating energy is produced in a heat source unit disposed outside a structure. Water, antifreeze, or the like is heated or cooled by a heat exchanger disposed in an outdoor unit and it is carried to an indoor unit, such as a fan coil unit or a panel heater, to perform heating or cooling (refer to Patent Literature 1, for example).
Moreover, an air-conditioning apparatus called a waste heat recovery chiller is constructed such that a heat source unit is connected to each indoor unit through four water pipes arranged therebetween and, for example, cooled water and heated water are simultaneously supplied so that cooling or heating can be arbitrarily selected in the indoor unit (refer to Patent Literature 2, for example).
In addition, an air-conditioning apparatus is constructed such that a heat exchanger for a primary refrigerant and a secondary refrigerant is disposed near each indoor unit to carry the secondary refrigerant to the indoor unit (refer to Patent Literature 3, for example).
Furthermore, an air-conditioning apparatus is constructed such that an outdoor unit is connected to each branching unit including a heat exchanger through two pipes to carry a secondary refrigerant to an indoor unit (refer to Patent Literature 4, for example).